Just jump on your computer and search “find my phone” in Google. You’ll see a map pop-up, revealing where you left your Android phone. You can even ring it remotely straight from your browser if it’s nearby but out of sight. The only thing you can’t do is wipe or lock your device if you think it’s been stolen, though that’s still an option using the Android Device Manager.
The entire process should happen automatically, assuming you have the latest version of Google’s app running on your smartphone. That’s the beauty and convenience of having Google everywhere all the time.